1. Field of the Invention
The present invention relates to a device for anchoring the end of at least one tensioned cable or bar, comprising an anchoring head having at least one conical bore through which said cable or said bar is passed, and a set of conical keys located within the bore in order to clamp said cable or said bar therein.
2. Description of the Prior Art
Anchoring devices of the aforementioned type are well-known and are described, for example, in French patent No. FR - 2,541,339. Such devices are employed in particular for prestressing concrete beams. It is necessary for this purpose to apply a considerable tractive force on one end of at least one cable which is encased in a sheath and follows a predetermined path within the beam to be prestressed, the opposite end of said cable or cables being anchored to one end of the beam by means of an anchoring device. When the cable or cables are under tension, that end of the cable or cables on which the tractive force is applied is also anchored to the corresponding end of the beam by means of a second anchoring device. With this objective, the tractive force applied to the cable or cables is relieved to a slight extent and the cable or cables displace the conical keys by frictional contact. The keys are thus jammed and have the effect of locking the cable(s) in position within the conical bore or bores of the anchoring head which is applied against the corresponding end of the concrete beam by means of a bearing plate in order to maintain the cable or cables under tension. Under service conditions, the conical keys remain jammed within the conical bores of the anchoring head under the action of the tensioned cable or cables. It is quite clearly a matter of primary importance to ensure that, when the prestressed beam is in service, the anchoring devices are not liable to release the cable or cables as a result of loosening of the conical keys since the initial tension applied to the cable(s) would otherwise be liable to decrease or even to fall to zero. Such a situation may arise if the cable or cables themselves become slack, for example as a result of failure of a cable strand, as a result of a violent impact applied to the concrete structure or as a result of an earthquake. A substantial reduction in tension of the cable or cables may in fact cause loosening of the conical keys and consequently a movement of relative displacement or slippage between the keys and the cable or cables. This type of situation is not usually attended by any adverse consequences if, after completion of the anchorages, concrete grout has been injected into the sheath which surrounds the prestressed cable or cables since, in this case, the cable or cables are securely maintained by the concrete which bonds them to the sheath and to the concrete structure. On the other hand, the situations mentioned above could be attended by much more troublesome consequences if no mass of concrete grout is injected into the sheath or in cases where the concrete beam is a hollow beam.
It is also a known practice to make use of anchoring devices comprising one or a number of sleeves which are fixed by drawing on the cable or cables to be anchored and which are applied either directly (passive anchoring) or by means of a nut (active anchoring) against one face of a bearing plate provided with at least one opening through which the cable or cables are passed, the other face of said bearing plate being applied against the concrete structure (U.S. Pat. No. 3,559,270). Known anchoring devices of this type do not suffer from the aforementioned disadvantages attached to known devices comprising conical keys but are more complicated to employ in practice since the sleeve or sleeves to be drawn must be very accurately positioned on the cable or cables at the moment of drawing.